The present invention relates to an optical disk which is operated with two different wavelengths of a short wavelength and a long wavelength.
An optical disk memory which achieves reproducing or recording/reproducing of information by irradiation of a light beam has been put into practical use as a storage medium for audio, video, and computer data files which realizes a large capacity, high-speed access, and portability. The optical disk memory is therefore expected to develop more in the future. High-density techniques for an optical disk are considered to involve shortening of the wavelength of a gas laser for cutting a master disk, shortening of the wavelength of a semiconductor laser as an operating light source, improvement of the numerical aperture of an objective lens, and thinning of an optical disk. Further, in a recordable optical disk, there are various approaches such as mark length recording, land-groove recording, and the like.
As a technique capable of effectively achieving high-density of an optical disk, a proposal has been made as to a super-resolution reproduction technique which utilizes medium films. The super-resolution reproduction has originally been proposed as a technique peculiar to a magneto-optical disk. Thereafter, reports were made as to attempts to provide a super-resolution film whose transmittance changes due to irradiation of a reproduction beam in the entering side of the reproduction beam with respect to a recording layer to perform super-resolution reproduction in case of a ROM disk in addition to magnet-optical disk. Thus, it has been revealed that the super-resolution reproduction technique is applicable to all optical disks including a magneto-optical disk, CD-ROM, CD-R, WORM (write once read many), phase change optical disk, and the like.
Super-resolution reproduction techniques are classified into a heat mode system and a photon mode system. Explanation will be made as for examples of conventional super-resolution films which have already been proposed.
In the heat mode system, a phase change material is used as a super-resolution film. The super-resolution film is heated by irradiation of a reproduction beam to cause a phase change, so that an optical aperture smaller than the reproduction beam spot is formed. The shape of the optical aperture follows the isotherm of the super-resolution film. However, since the size of the optical aperture easily changes due to influences from the environmental temperature, it is necessary to perform heat control strictly in compliance with the linear velocity of the optical disk. In addition, it is difficult to obtain sufficient repetition stability in the super-resolution film of the heat mode system because of thermal fatigue caused by reproduction and recording.
In the photon mode system, a photochromic material is used as a super-resolution film, and development or erasure of color based on irradiation of a reproduction beam is utilized to form an optical mask or an optical aperture. In the photochromic material, electrons are excited from a ground level to an excited level of short lifetime by irradiation of light, and further, electrons are transferred from the excited level to a metastable excited level of very long lifetime in which they are captured thereby to cause change in absorption characteristics. Therefore, in order to perform reproduction repeatedly, electrons captured at the metastable excited level must be deexcited to the ground level so that the optical aperture may be closed. However, since a supplemental beam is irradiated for the deexcitation, two-beam operation is required disadvantageously in view of high-speed response. Also, the photochromic material causes transmittance change through complicated processes accompanying atomic migration or change in molecular bonds, and therefore, the repetition stability is limited up to about 10,000 times.
As described above, to realize super-resolution reproduction of an optical disk, it is required that the transmittance change of the super-resolution film is caused within the range of practical reproduction power, the degree of change is large, an optical aperture can be formed within a short time equivalent to about the passing time of the reproduction beam spot, and repetitive reproduction can be performed many times.
Furthermore, shortening of the operating wavelength is essential for high-density of an optical disk. This is for the reason that by achieving the shortening of the wavelength of a semiconductor laser as an operating light source, the spot size of a recording beam becomes small, and recording marks can be formed at a narrow pitch. For example, under the present condition, recording, reproduction, and erasure of DVD-ROM and, DVD-RAM are performed by using a semiconductor laser with a wavelength of about 640 nm. However, to achieve a higher-density in future, there is a large possibility to use a blue semiconductor laser with a wavelength of 410 nm.
Suppose that a recording mark train with a narrow pitch is recorded on an optical disk by using an optical disk drive with a light source wavelength of 410 nm of the future, and this optical disk is reproduced by using an existing optical disk drive with a light source wavelength of 640 nm. In this case, since a reproduction beam with a large spot size is irradiated onto the marks with a narrow pitch, inter-symbol interference becomes large, and it is difficult to obtain a sufficient intensity of the reproduction signal.
Accordingly, even if an optical disk is adjusted so that the recording can be performed by light with a short wavelength, in order to realize an optical disk which operates with a plurality of wavelengths, the recording must be performed at a wider mark pitch than the mark pitch which can be realized by light with a short wavelength to obtain a sufficient reproduction signal by light with a long wavelength, and there is such a problem that the recording density is limited. Thus, it is difficult to provide a compatible optical disk to a plurality of operating wavelengths.
An object of the present invention is to provide an optical disk which can read out information recorded by light with a short wavelength at a narrow pitch, by light with a long wavelength.
According to the present invention, there is provided an optical disk which can operate with at least two different wavelengths, comprising a disk substrate on which tracks are formed at a track pitch corresponding to light with a short wavelength, a super-resolution film whose transmittance changes due to the irradiation of light, and a recording layer, wherein the transmittance of the super-resolution film changes due to irradiation of light with a long wavelength.
In another optical disk of the present invention, the transmittance of the super-resolution film changes due to the irradiation of light with a long wavelength and the irradiation of light with a short wavelength respectively.
In still another optical disk of the present invention, the super-resolution film has a stacked structure comprising a super-resolution film whose transmittance changes due to the irradiation of light with a long wavelength and a super-resolution film whose transmittance changes due to the irradiation of light with a short wavelength.
In the super-resolution film according to the present invention, it is preferable that a region where the transmittance changes due to the irradiation of light with a long wavelength is narrower than a region where the transmittance changes due to the irradiation of light with a short wavelength. Here, the region where the transmittance changes, means the region where the transmittance becomes higher than the initial transmittance, that is, the region (size of an optical aperture) corresponding to the number of irradiated photons larger than the threshold value in the increase of the transmittance accompanied with the number of irradiated photons.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.